The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same.
The answer is D) enzymes.
“Enzymes are protein catalysts. A specific region of an enzyme where a substrate binds and catalysis takes place.”
It is a perfect example of a natural selection.
(I'm not sure if this is the exact answer that you are looking for.)